The present invention relates to multiplexers and de-multiplexers for wavelength division multiplexed optical communications systems. More particularly, the present invention relates to multiplexers and de-multiplexers that utilize diffraction gratings to either separate or combine optical channels carried in such optical communications systems.
In a wavelength division multiplexing optical communication system, information is carried by multiple channels each with a distinct wavelength. It is often necessary to separate, combine, selectively attenuate or amplify each wavelength channel. In order to perform these functions one usually needs to spatially separate the wavelength channels from one another. In this document, these individual information-carrying lights of a wavelength division multiplexing optical fiber, optical line or optical system are referred to as either xe2x80x9csignalsxe2x80x9d or xe2x80x9cchannels.xe2x80x9d The totality of multiple combined signals, wherein each signal is of a different wavelength range, is herein referred to as a xe2x80x9ccomposite optical signal.xe2x80x9d Although each information-carrying channel actually comprises light of a certain range of physical wavelengths, for simplicity, a single channel is referred to as a single wavelength, xcex, and a plurality of n such channels are referred to as xe2x80x9cn wavelengthsxe2x80x9d denoted xcex1-xcexn.
A de-multiplexer is an apparatus that receives a composite optical signal comprising a plurality of wavelengths or channels and separates the channels among a plurality of respective outputs. A multiplexer is an apparatus that receives a plurality of wavelengths or channels from separate respective inputs and combines them into a single composite optical signal directed to a single output. Because light paths are generally reversible through most optical components and apparatuses, such separation or combination can generally be performed by a single apparatus, depending upon the direction of light through the apparatus. Such an apparatus that can be utilized as either a multiplexer or a de-multiplexer is herein termed a multiplexer/de-multiplexer (MUX/DEMUX).
A diffraction grating is an effective wavelength dispersive component that can be used to separate wavelength channels. It is known that the resolving power of a diffraction grating not only depends on angular dispersion of the grating setup, it also depends on the size of the optical beam that is incident on the grating. The resolving power can be written as:       λ          Δ      ⁢              xe2x80x83            ⁢      λ        =  mN
where xcex is center wavelength, xcex94xcex is the minimum wavelength difference that can be resolved, m is diffraction order, and N is the number of xe2x80x9cgroovesxe2x80x9d that are illuminated by the incident optical beam. Apparently, because N is proportional to the width of the incident optical beam, the resolving power is linearly proportional to the width of the incident beam.
It is also necessary for the divergence of an optical beam incident upon the grating to be smaller then the angular dispersion provided by the grating to effectively separate two adjacent wavelength channels. If a lens is used to collect the diffracted beams and focus them to different respective spots, the resolving power expression also means that the diffracted beam of a wavelength channel must be focused into a spot that is smaller than the spatial separation of two adjacent wavelength channels at the focal plane.
Fiber collimators are often used to collimate divergent optical beams emerging from optical fibers. Because an optical beam emitted from a single-mode fiber is circular, the collimated beam possess a round cross section. The diameter of the collimated beam is proportional to the focal length of the collimating lens. To achieve large beam size so as to illuminate an adequate number of grating grooves, one can choose a long focal length collimating lens. However, if this is done, subsequent optical systems must also be designed to handle large beams. This usually creates an increase in system complexity and overall size.
It is realized by the inventors of the present invention that, that for most fiber optics applications, the incident beam only needs to have a large width in the dispersive direction of the gratingxe2x80x94that is, perpendicular to the grating xe2x80x9cgroovesxe2x80x9d or other diffraction-causing pattern on the grating. This means that an anamorphic optical beam with an elliptical cross-section can be utilized to achieve high spectral resolution while preserving compact system size and simplicity. There is therefore a need, in the art, for a grating-based MUX/DEMUX that can utilize anamorphic optics and anamorphic beams to minimize device size while maintaining adequate resolving power.
To address the above-mentioned need, an apparatus and a method for an improved grating multiplexer/de-multiplexer (MUX/DEMUX) apparatus are herein provided. The present invention discloses an apparatus and a method that employs anamorphic or elliptical optical beams to achieve high spectral resolution and yet compact device size. In a first preferred embodiment, a MUX/DEMUX apparatus in accordance with the present invention comprises an optical device providing multi-channel input light, a collimator optically coupled to the optical device, an anamorphic pair of prisms optically coupled to the collimator opposite to the optical device, a diffraction grating optically coupled to the anamorphic pair of prisms at a side opposite to the collimator and a focussing lens optically coupled to the diffraction grating.
In a second preferred embodiment, a MUX/DEMUX apparatus in accordance with the present invention comprises an optical device providing multi-channel input light, a collimator optically coupled to the optical device, an anamorphic pair of prisms optically coupled to the collimator opposite to the optical device, a diffraction grating optically coupled to the anamorphic pair of prisms at a side opposite to the collimator, a focussing lens optically coupled to the diffraction grating and a micro-mirror array optically coupled to the focusing lens at a side opposite to the diffraction grating, wherein each micro-mirror comprising the micro-mirror array is disposed at a focal point of a different respective channel.
In operation of a grating MUX/DEMUX in accordance with the present invention as a de-multiplexer, a light comprising a composite optical signal received from the optical device is collimated into a beam with a circular cross section by the collimator and expanded substantially only along one dimension by the anamorphic pair of prisms. The resulting anamorphic light beam with elliptical cross section is then directed to the diffraction grating whereat it is separated, by diffraction, into its component channels. The separated, anamorphic or elliptical light beams comprising the diffracted channels are then focused to a plurality of respective focal points by the focussing lens.
Operation of the same apparatus as a multiplexer occurs when a plurality of channels are input to the apparatus from optical devices disposed at the locations of the respective focal points.